When a volume of saline is diluted with an equal volume of freshwater, the entropy of the system is increased. This increase in entropy can be considered to arise from the more random location of the salt ions: initially constricted to the volume of saline, the ions can be located in twice as many locations after dilution with an equal volume of freshwater. This increase in entropy can be harnessed to do work. Vast amounts of freshwater and wastewater mix with saline as they flow into the ocean. The energy released by this mixing represents a large, renewable, green, and available power source.
For example, complete mixing of freshwater with brine whose salinity equals ocean water (˜800 mM) provides free energy of 2.5 kJ/1 or 0.82 Wh/kg. The energy released by this mixing represents a power source that could be expanded globally to capture the 2 TW generated by the flux of fresh river and wastewaters into the ocean.
The key challenge to exploiting the mixing of freshwater and ocean water is the low energy density: 4 watt-hours per gallon of freshwater (about 0.01% of the energy in a gallon of gasoline). However, the volume of available freshwater and wastewater is so enormous that the power available from mixing freshwater and ocean water represents a sizeable fraction of the U.S. energy production, about as large as current nuclear power. Thus, the technical challenge is to extract this energy with sufficient efficiency to make this vast power source economically viable.
Generating electrical power by selective diffusion of ions from saline to freshwater is termed reverse or inverse electrodialysis. Current state-of-the-art techniques entail the separation of the saline and freshwater by membranes that are selectively permeable to cations or anions. The separate diffusion of cations and anions from saline into separate compartments of freshwater separates charge, creating electrical potential and the capacity to do work. Reverse electrodialysis is feasible, but the extraction efficiency is low, only about 40 mW per square meter of membrane.
Currently deployed technologies for reverse electrodialysis involve membranes that are selectively permeable to cations and anions. Although power plants using this technology are being built overseas, several factors limit the rate of power generation and thus the economic viability: high freshwater impedance, polarization, electrode consumption, membrane cost and biofouling, and DC current. These problems have prevented the large-scale utilization of reverse electrodialysis to generate electrical power.
High freshwater impedance. Ions that diffuse across the semipermeable membrane into freshwater must then diffuse across the freshwater to the electrode. The high impedance of freshwater substantially reduces the maximum current density. DC reverse electrodialysis maintains high impedance of the freshwater at the anode (Cl permeable membrane side) because this water does not become salinated (Cl− diffusing through the membrane is deposited on the anode).
Polarization. Power production in reverse electrodialysis is driven by ionic diffusion, and this can be rapidly diminished by resistive and concentration overpotentials at the electrodes and membrane caused by local imbalances of the ionic charge carriers. A critical issue for salination technology is that because the driving forces are so small, even very modest charge accumulations reduce rates of ion diffusion and power generation by over an order of magnitude without stirring, but stirring requires too much power. Technologies that have been proposed to circumvent the high impedance of freshwater remain limited by polarization and consequent overpotentials so that the power density of these devices is still only 0.1 W/m2, which is not economically viable.
Electrode consumption. Using Ag/AgCl2 electrodes as an example, DC ionic currents gradually deposit excess Cl on the anode and deplete the cathode of Cl. Cl is depleted from the cathode (in the sodium-accepting freshwater pool) as Cl goes into solution (2e−+AgCl2→Ag+2Cl−), and Cl is deposited on the anode (the Cl-accepting freshwater: Ag+2Cl−→AgCl2+2e−). This limits electrode life and necessitates moving electrodes or saline/freshwater solutions so that chloride shifts can be periodically reversed. More complex electrodes are being studied but these issues remain barriers to the feasibility of DC reverse electrodialysis. Current solutions to this problem involve toxic electrode half-reactions, electrode rinse solutions that partially short-circuit the battery, or both.
Membrane cost and biofouling. Current costs of semipermeable membranes are $100/m2, and this must be lowered by a factor of 30 before reverse electrodialysis can be an economically viable means of power production. Semipermeable membranes used for industrial purposes are prone to fouling by bacterial biofilms that reduce the rate of ion diffusion. The current rate of degradation of power production due to biofouling is 70% every 6 months at the WETSUS reverse electrodialysis pilot power plant. The maintenance costs of these membranes are currently an important limitation to economic feasibility of useful power generation from reverse electrodialysis.
DC current. Current reverse electrodialysis technologies produce DC current that must be converted to AC with attendant losses.